Are you sure you need to? As the speaker is part of the sound, you might well be better off miking the amp. A SM57 is good for this; experiment with position (pointing to side or centre of speaker, distance from speaker). Or a nice cheap Behringer C2, if your mixer has phantom power.

Breaking into the amp's electronics is fraught with unknowns. If you have to ask the question, you probably aren't competent (no offence intended) to do it... but if I needed to make such a mod, I'd try picking up a signal from the input to the master volume control potentiometer, assuming it's wired in the conventional way as a simple attenuator and not as part of a feedback loop.

I take no responsibility if you try this and something bad happens.

Anahata